A multilevel modulation technology is known as a means of transmitting information including plural bits per symbol in order to increase a transmission quantity per second in a radio-communication system.
In the multilevel modulation technology, as the bit number per symbol increases, the maximum throughput increases when the quality of a propagation path is good, but in contrast the throughput largely decreases when the quality thereof lowers. Therefore, in order to guarantee a stable communication, an adaptive modulation technology is proposed wherein the number of modulation levels is changed in proportion to the quality of a propagation path. The technology is explained in the paper “The transmission characteristics of a modulation-level-number-variable adaptive modulation method” (The Transactions of the Institute of Electronics, Information and Communication Engineers, B-II, Vol. J78-B-II, No. 6, pp. 435-444, June 1995) and others.
Further, with the expanding adoption of wideband in radio-communication, a multi-carrier communication method called OFDM (Orthogonal Frequency Division Multiplexing) is used wherein information is transmitted by dividing and allocating it to plural orthogonalized subcarriers and an OFDM adaptive modulation method is proposed wherein a modulation method is changed in accordance with each subcarrier in order to cope with the variation of a propagation path quality within the band of OFDM. The technology is explained in the paper “An OFDM adaptive modulation method using multilevel transmission power control for high-speed data communication” (The Transactions of the Institute of Electronics, Information and Communication Engineers, B-II, Vol. J84-B-II, No. 7, pp. 1141-1150, July 2001) and others.
In the OFDM adaptive modulation method, the variation of a propagation path quality can be followed within a band by changing a modulation method in accordance with each subcarrier, but in contrast a control signal is required to conform the modulation method of a subcarrier at a transmitting station to that of the same subcarrier at a receiving station, which causes complication. To cope with that problem, a technology of avoiding the complication by grouping some subcarriers and changing a modulation method in accordance with each subcarrier group is explained in the paper “An efficient blind modulation detection algorithm for adaptive OFDM systems” (IEEE Vehicular Technology Conference (VTC), Oct. 4-9, 2003).
In the aforementioned method wherein a modulation method is changed in accordance with each subcarrier, though the variation of a propagation path quality in each band can be followed, the problem exists that a control signal is additionally required to conform a modulation method at a transmitting station to that at a receiving station. Further, in the aforementioned adaptive modulation method wherein a modulation method is changed in accordance with each subcarrier group, though the advantage is that the control signal to conform a modulation method at a transmitting station to that at a receiving station can increasingly be reduced as the number of subcarriers belonging to a group increases, the problem exists that the variation of a propagation path quality in each band cannot be followed.